This invention relates generally to electro-dynamic laser systems, and, more particularly, to an electro-dynamic laser system which is capable of controlling the optical beam mode thereof by the utilization of an injected laser beam in conjunction with a specifically designed diffraction grating.
Since the development of the first working lasers, considerable time and effort has been expended in the search for high power output laser systems. The possible applicatons of high power lasers are unlimited in the fields of communication, manufacturing, construction, medicine, space exploration, and defense.
The gas laser has grown out of the initial laser effort and is representative of one of the more sophisticated laser techniques which has the capability of providing very high power radiation output, due primarily to the large gas handling capability characteristic of such a system and due to the large quantity of energy which can be added to the gases flowing in such systems.
While the preferred embodiment of the present invention will be described in connection with an electrically excited nitrogen (N.sub.2), carbon dioxide (CO.sub.2) and helium (He) laser, it may be applied to other systems where a conducting ionized gas is required or useful and including, but not restricted to, gas constituents other than N.sub.2, CO.sub.2 and He as well as other lasing systems.
In order to bring about laser action two conditions must be fulfilled: (1) population inversion must be achieved and (2) an avalanche process of photon amplification must be established in a suitable cavity or resonator such as, for example, an optical cavity, optical resonator or resonant cavity. Population inversion can, for example, be accomplished if (1) the atomic system has a least three levels (one ground and at least two excited levels) which can be involved in the excitation and emission processes and (2) the lifetime of one of the most energetic of the excited states is much longer than that of the other or others.
When a system is in a condition where light (photon) amplification is possible, laser action can be achieved by providing (1) means for stimulating photon emission from the long-lived state, and (2) means for causing photon amplification to build up to extremely high values. In the usual embodiment, this is accomplished by fashioning the medium containing the active atoms into a chamber with perfectly (as far as possible) parallel ends polished so highly that the surface roughness is measured in terms of the wave length of the laser. The ends may be simply polished metal or they may be silvered or dielectric coated so that they behave as mirrors which reflect photons coming toward them from the interior of the chamber. Such a structure, whether the mirrors are within or outside the chamber, is called the resonator, that is, the optical or resonant cavity.
If now pumping means, such as for example, an electric discharge acts on the medium and brings about population inversion of the long-lived state with respect to another lower energy excited state even though the long-lived state is only relatively long-lived, in a small fraction of a second there will be spontaneous emission of photons. Most of these photons will be lost to the medium but some of them will travel perpendicular to the ends and be reflected back and forth many times by the mirrors. As these photons traverse the active medium, they stimulate emission of photons from all atoms in the long-lived state which they encounter. In this way the degree of light amplification in the medium increases extraordinarily and because the photons produced by stimulated emission have the same direction and phase as those which stimulate them, and assuming the optical quality of the laser media is suitable, the electromagnetic radiation field inside the chamber or cavity is coherent. In order to extract a useful beam of this coherent light from the cavity, one (or both of the mirrors is made slightly transmissive. A portion of the highly intense beam leaks through the mirror, and emerges with regularly spaced wave fronts. This is the laser beam.
In the electro-dynamic laser an electron beam is fired into a gas filled optical or resonant cavity so as to provide electrons. The use of an electron beam for laser pumping is fully described in U.S. Pat. No. 3,702,973 issued Nov. 14, 1972. These electrons are subject to the sustainer voltage which adds energy to them, heating them to a desired temperature. In the case of the CO.sub.2 laser, the electrons transfer some of their energy to N.sub.2 and CO.sub.2 in the cavity by collision processes, pumping (quantum mechanically) these gases to an upper laser level. The N.sub.2 transfers its energy to the CO.sub.2. The CO.sub.2 relaxes to a lower level with the emission of radiation. The cavity is bounded with mirrors which reflect some of the simulated emission back into the cavity stimulating more emission, etc. The radiation is eventually led out of the cavity in the form of a laser beam.
To date, the means to modify the gain-switched spike turn-on signal, in the electro-dynamic laser system has been accomplished by injecting an external beam into the resonator or resonant cavity through a small hole drilled in one of the cavity mirrors such as described in a paper entitled "Gain Spiking and mode-beating control by signal injection in CO.sub.2 lasers" by Charles Cason et al, Journal of Applied Physics, vol. 48, June 1977, pgs 2531-2536. With such an approach, one is limited to low injection powers and to irradiating only a small area of the resonator gain region therefore substantially reducing the reliability of mode control.